Social Question

Why do jeans tear up underneath?

For years, I have jeans which tear up below the crotch area. Is this a normal wear and tear situation? Is it possible to avoid? I have less than two year old jeans that end up with a hole right away. I wash them once a week.

What I do sometimes is tie the pants in a single knot assuming it would prevent its undercarriage from being overly stretched. Thanks.